1. Field of the Invention
The present invention relates to a method of preparing polyimide foam with excellent flexibility properties and more particularly, to a method of preparing polyimide foam with excellent flexibility by foaming polyimide precursor which is prepared by employing linear aliphatic diamine and aromatic diamine containing 4 to 12 carbon atoms, at specific weight ratio into the esterified reaction mixture prepared by reacting an aromatic carboxylic acid or its anhydride with excess amount of an alcohol compound.
2. Description of the Prior Art
"Polymer foam" refers to a light substance in which the cells within the polymer are well distributed with low density. The foam is widely used as adiabatic, cushion, shock absorbent, soundproof and packing materials due to its excellent cushion, adiabatic, soundproof and vibration-proof properties. The general purpose foams such as polyurethane or polyolefin foams are widely used in transportation equipment such as automobile and ships due to their superior physical properties such as strength, restoring capacity and adiabatic and economic efficiency. However, for a use in aerospace, submarine, special ships and express train, which require superior heat-resistance and flame retardancy, a special heat-resistance foam with superior heat-resistance and flame retardancy properties is used therein. With an aromatic or heterocyclic structure, typical heat-resistance foams include polyimide, polyisocyanurate and polybenzimidazole with high heat-resistance. In particular, the polyimide foam is known to have a wide application due to its superior thermal stability and flame retardancy.
Methods for preparing polyimide-based foams are disclosed in U.S. Pat. Nos. 4,241,114, 4,241,193, 4,273,886, 4,296,208, 4,305,796 and 4,332,656. These methods include a foaming by means of heating a polyimide in an oligomer condition in an oven or microwave oven. However, the problem of a difficulty in control of the physical properties arose in which a chain extension reaction and foaming reaction occur simultaneously.
Further, U.S. Pat. No. 4,319,000 discloses that 3,3 ',4,4'-benzophenone tetracarboxylic acid dianhydride (BTDA) and ethyl alcohol as esterifying agent were used to control amount of the closed cell to be below 95% and two types of aromatic amines having a base dissociation constant lower than 10.sup.-10 were selected from the group consisting of 4,4'-diaminodiphenylsulfone, 3,3'-diaminodiphenylsulfone, and 2,6-diaminopyridine (2,6-DAP).
U.S. Pat. No. 4,369,261 discloses that BTDA, methylenedianiline (MDA), 2,6-DAP, and an activated carbon or graphite with superior conductivity were used to prepare a polyimide foam with improved flexibility, homogeneous cell structure.
U.S. Pat. Nos. 4,647,597, 4,656,198 and 4,670,478 disclose that using BTDA, MDA, and 2-methoxyethanol as a plasticizer were used to prepare a polyimide foam.
U.S. Pat. Nos. 4,806,573, 4,824,874 and 4,830,883 disclose not only the method for preparing a polyimide precursor by reacting BTDA, organic polyisocyanate and furfuryl alcohol but also a way of using precursor as a reinforcement agent of a foam with an open cell structure.
U.S. Pat. No. 4,952,611 discloses that a polyimide foam with superior compression set property for a use in seat cushion, was prepared by using a mixture of BTDA and 2-(vicynal-dicarboxycyclohexenyl)succinic acid in addition to an amine selected from methylenedianiline or 2,6-diaminopyridine.
U.S. Pat. No. 4,978,692 discloses that 3,3',4,4'-benzophenonetetracarboxylic acid and an amine selected from the group consisting of 4,4'-bis[4-(3-aminophenoxy)phenyl]sulfide, 4,4'-bis[4-(3-aminophenoxy)phenyl]sulfone, and 4,4'-bis(3-aminophenoxy)benzophenone were used in order to obtain a cell size of 0.1 to 1.0 mm in a polyimide foam.
U.S. Pat. No. 5,234,966 discloses a method of preparing a polyimide foam of desired density depending on the water content by reacting BTDA, MDA, 2,6-DAP, and the mixture of alcohol compound and water as a esterifying agent.
Polyimide foams prepared by previous methods show some physical improvement in heat-resistance, flame retardancy, but still nearly no improvement in flexibility.
Therefore, it is required to provide an improved method for preparing a polyimide foam with flexibility as well as its physical properties such as flame retardancy and heat-resistance.